Pickproof lock construction



Sept. 26, 1939.

s. BRANTINGSON PICKPROOF LOCK CONSTRUCTION Filed Nov. 3, 1938 v INVENTOR 5/6020 BEA/VT/NGSOA/ Patented Sept. 26, 1939 PATENT OFFICE PICKPROOF LOCK CONSTRUCTION Sigurd Brantingson, Palisades Park, N. J., assignor to Francis Keil & Son, Inc., a corporation of New York Application November 3, 1938, Serial No. 238,515

11 Claims.

This invention relates to lock construction and more particularly to a lock construction of the pin tumbler type.

One of the objects of this invention is to provide a lock construction that will effectively resist picking and which in construction will be free from costly complications. Another object is to provide a pickproof lock construction of the pin tumbler type which will be inexpensive to manufacture and which so far as manufacture is concerned may be easily made to follow routine manufacturing processes as employed in the present-day low cost type of cylinder or pin tumbler lock manufacture. Another object is to provide a pickproof lock construction that will be inexpensive to manufacture, that makes it possible to avoid peculiarly shaped and expensive parts, and that will make it possible to employ existing methods of assembly. Another object is to provide a simple and dependable pin tumbler lock construction capable of utilizing simple and inexpensive forms of pins or pin tumblers and yet constructed and operating to resist effectively attempts to pick it.

Another object is to achieve in a lock of the pin tumbler type genuine proof against picking without having to import into the construction complicated modifications of structure or costincreasing additional parts and thus to make available to the public a low cost but genuinely pickproof lock construction. Many attempts have heretofore been made to produce a pin tumbler lock construction incapable of being picked In some of these attempts, so-called mushroom or headed upper or driver pins have been employed sometimes throughout the lock construction and sometimes in combination with straight or cylindrical or with cylindrical but end-tapered driver or upper tumbler pins and, in fact, struc tures according to such prior attempts are today on the market, but all of them can be actually picked and they simply increase to a small extent the time and difliculty in picking the look as against the time and difficulty involved in pocking the ordinary lock in which all of the pins are cylindrical.

Other attempts have been made by utilizing one or more mushroom or headed driver or upper tumbler pins and even also one or more mushroom or headed pins in the key plug, in coaction with added moving parts in the form of disks, slides, sleeves, or the like, or other additional part or parts intended to partake of movement relative to the casing and the key plug and to the necks or heads of the mushroom pins, in the endeavor to achieve proof against picking, but in such instances, aside from other shortcomings, the structural complications are so great that either cost of production and hence cost to the consumer is prohibitively high or there results such unreliability of normal or other action as to forestall commercial success.

Other attempts disclosed in the prior art include the use of a long pin in the key plug, usually at the front end of the latter, dimensioned as to length so that the insertion in the key slot of a picking too-l lifts the pin into the tumbler recess or bore contained in the lock casing, the intention being that thereby such relative displacement, as by applyinga torsional or twisting force on the key plug, between the casing and key plug, as is necessary to catch or hold other tumbler pins, is prevented; but such structures of the prior art can be easily picked and the latter characteristic doubtless accounts for the fact that, though such structures have been long disclosed or known in the prior art, they are not on the market.

One of the .dominant aims of this invention is to avoid and overcome such commercially fatal and other deficiencies and disadvantages as those pointed out above, and to produce a low cost, dependable, secure and durable lock construction that effectively precludes picking. Other objects will be in part obvious or in part pointed out hereinafter.

The invention accordingly consists in the features of construction, combinations of elements, and arrangements of parts as will be exemplified in the structure to be hereinafter described and the scope of the application of which will be indicated in the following claims.

In the accompanying drawing in which is shown by way of illustration one of the various possible mechanical embodiments of my invention,

Figure l is a central vertical longitudinal sectional view through the lock construction;

Figure 2 is a transverse sectional view as seen along the line 2-2 of Figure 1;

Figure 3 is a transverse sectional view as seen along the line 3-3 of Figure 1;

Figure 4 is a fragmentary side elevation of the key plug. and

Figure 5 is a sectional view like that of Figure l but on a smaller scale and fragmentary, showing the relationship of the parts with a proper key inserted into the lock construction.

Similar reference characters refer to similar parts throughout the several views in the drawing.

Referring first to Figure l, I have there shown at lfl a lock casing having a lower casing portion ll preferably cylindrical (see Figures 2 and. 3) and bored out, preferably also cylindrically, as at 12, for movably receiving therein a key plug l3, preferably cylindrical, and neatly fitted into the bore l2 to partake of movement relative thereto. The key plug and the casing are provided each with bores or recesses for the reception of upper and lower tumbler pins, all. in the preferred manner as later described, the casing ll] having an upper extension or casing portion M in which the bores CB, preferably cylindrical, are formed, the bores being preferably cylindrical and being preferably drilled with their axes preferably parallel and falling in a plane that preferably passesthrough the axis of the bore I2 and hence also through the axis of the key plug l3.

In the key plug I3 is a key slot l5 which may take any suitable or appropriate form, illustratively like that shown in Figures 2 and 3, being preferably of generally rectangular cross-section but longitudinally ribbed, thus to adapt it to take a flat key suitably grooved longitudinally to interfit with the ribs of the key slot. Thus, the upper portion may have a plurality of V-shaped ribs R. arranged, for example, two on the lefthand side and one on the right-hand side and the intermediate or rest of the rectangular cross-section may have two spaced ribs l1 and 23 on the right-hand side and an intermediate rib IE on the left-hand side. Rib I! may have a substantially horizontal under face and rib I6 may have a substantially horizontal upper face, and these two faces are vertically displaced.

In the key plug I3 I provide pin tumbler bores PB preferably with their axes parallel and coincident generally with the central vertical plane of the key slot I5, the latter being preferably alined along the central vertical plane of the key plug l3 with the latter positioned in its normal or locked position.

The key plug bores PB are preferably formed by drilling and may be of any suitable number, illustratively six, and of any suitable length or lengths. The bores PB may be and preferably are of the same axial length, thus avoiding complications in manufacture. As better appears from Figures 2 and 3, the bores are drilled so as to intersect the upper ribs R and they preferably But for achieving freedom from binding and ease of turning movement, it is preferable to allow for a slight play in an axial direction, particularly also because of necessary tolerances in manufacture and assembly. Accordingly, the flange l8 and ring 20 may permit a relative axial movement between casing and key plug of several thousandths of an inch, for example three or four thousandths.

In the preferred construction, the diameters of 1 the casing bores CB and key plug bores PB are the same and preferably uniform throughout; illustratively, they may be of a diameter of 0.123 inch.

In the casing tumbler bores CB I provide casing tumblers preferably in the form of cylindrical pins and preferably of the same length but each of a diameter preferably materially less than the bore in which it is received. Thus, where the bores CB are all of the same diameter 2 and of 0.123 inch by way of the above illustration, these casing tumbler pins E E E E, E and E are preferably of the same diameter and illustratively of 0.115 inch. This difference in the diameters includes a factor adequate to compensate for the above-mentioned end play or relative axial play between key plug and casing and insures that the pins E E E E E under the action of the springs S are entered into the key plug bores PB, even though this end play results in a slight misalinement between key plug bores and casing bores. The casing bores CB may be closed off as by a plate 2i.

Coacting with the tumbler pins E E E E and E are key plug tumbler pins K K K K K respectively; these may be of differing lengths according to the particular coding selected for the lock and for the corresponding key and each preferably matches the diameter of its coacting casing tumbler pin. In the illustrative case, these key plug pins K to K are therefore preferably 0.115 inch in diameter. Here again, allowance is included in the .dimensional difference for possible end play between key plug and casing and thus the entry of the pins K to K into the casing bores 013, under the action of a key, is assured even though the respective upper and lower bores might be slightly out of line may. These tumbler in H l p stantially tangent to the uppermost end of the cylindrical wall of the bore PB, whence it extends downwardly and toward the center line of the bore PB, intersecting the bottom of the plug bore PB along the diameter of the latter and extending sufficiently downwardly, as viewed in Figures 4 and 2, so as substantially to terminate at or preferably slightly below the upper face of the rib [6, thus also intersecting the key slot l5 as well as the bore PB of pin K Plate P is of a thickness on the order of several thousandths less than the width of the slot 22, so that it may freely move in the slot and where the slot 22 has an arcuate bottom 24, the bottom edge 25 of plate P is likewise and preferably arcuate, so that it may snugly bottom in the slot 22 even though slightly displaced to the left or right of the center line; the plate P is of a length, as viewed in Figure 2, such that its arcuate bottom edge 25 is somewhat less than the arcuate length of'the bottom 24 of the slot 22, and when bottomed in the latter it closes off the key slot portion I5 that is to the left of the rib l1 and itlalso closes off that portion of the key slot that is above the horizontal plane through the upper face of the rib 16, the matching curvatures of slot bottom and bottom edge 25 of the plate P insure that this closing off of these portions of the key slot dependably take place even though the plate isfshifted somewhat to the left or right of exact coincidence of center lines of plate and key'plug'. j

The upper edge of plate P is cut out to form a seat for the lower end of pin K and this seat may have a preferably straight line bottom edge 26 of a length equal to or slightly greater than the diameter of pin K bounded on each side by upwardly and outwardly inclined edges 21, 21.

In assembly, plate P is first slipped into the slot 22, thus bringing its seat-like edge 26 crosswise of the bore PB, whence the pin K is dropped in its bore, its lower end being dependably guided into contact with the seat 26 by the outwardly diverging guiding edges 21, 21, the reaction of the pin K on the latter edgesbein'g such as to nicely center the plate P.

With the plate P bottoming in the slot 22 and the pin K resting in its seat on the plate P the length of pin K is such that its upper end is flush with the outer cylindrical surface of the key plug (see Figure 1,) and it thus coincides with the line or plane of junction between key plug and casing bore.

In diameter, pin K is preferably larger than that of the illustrative equal-diametered pins K to K and preferably is only sufficiently smaller in diameter than the diameter of its own two bores PB and CB, as is necessary to compensate for the above-mentioned end or axial play between key plug and casing. With the above illustrative dimensions of other parts, this tumbler pin K is illustratively of a diameter of 0.118 inch. This difference between its own diameter and that of the casing bore CB with which it is to coact insures that it dependably enters the latter in spite of the above-describedend play. If it were possible atlow cost to eliminate such end play completely, theabove set forth illustrative dimensions could be correspondingly revised to avoid compensatingfor end play but the difference in diameters between tumbler pin K and the illustratively equally-diametered tumbler pins K to K would be retained, for a purpose presently to be described.

In normal or locked positionof the parts they appear as is shown in Figure 1. The junction between pins E and K coincides with the junction between key plug and easing bore so that neither of these two pins takes part in preventing relative movement between key plug and easing. But depending upon the coding of the key and other factors, the junctions between the remaining pairs of upper and lower tumbler pins are displaced from the junction between key plug and casing bore, being illustratively displaced downwardly therefrom in varying degrees, and thus the construction is held in locked position.

In such locked position, the construction cannot be picked, as explained below, and to unlock it, a flat key Y (Figure 5) having longitudinal lateral grooves to mate with the ribs R, I1, l6 and 23, is inserted into the key slot 15 in the usual way, the key having a cross-section to match the cross-section of the key slot I5; its upper edge is so shaped that the junctions between the pairs of tumbler pins E -K E K etc., are brought and held in coincidence with the junction between key plug and casing bore, as is indicated in Figure 5 with respect to the tumbler pins Bi -K while the junction between pins E and K is maintained in similar coincidence, the key Y when brought to rest having a notch therein, corresponding to the location of the pin K or of plate P, of such depth as is clearly indicated in Figure 5 that the plate bottoms in its slot 22 and the upper end of pin K is coincident with the above-mentioned junction. Thus. with such a key the lock may be unlocked.

The lower edge 25 of plate P is in transverse cross-section rounded off, as indicated in Figure 1, thus more smoothly to coact with the notched key edge. As the forward notches of the key lift up the plate P, pin K is also lifted up but lateral displacement of the plate P is prevented due to the action of the spring S and the projection of the lower end of pin K between the side walls 21, 21 of the seat.

Without the proper key, however, it has been found impossible to bring about an actuation of the lock and a better understanding of this result can be had by considering briefly such prior art constructions as heretofore mentioned which propose to employ a tumbler pin in the key plug, in the first bore thereof, and of a length equal to the key plug tumbler bore itself. The procedure in picking such a prior construction, and it is a matter of only a few minutes, is first to employ a suitable picking tool which is inserted into the key slot, thus raising the front tumbler pin and hence projecting its upper end into the tumbler bore of the casing; it can be held in such upwardly projected position by exerting a torsional strain on the key plug.

If there is no play or difference in pin and bore diameters, the picking tool may then be employed to force successively the other tumbler pins upwardly to bring their junctions in coincidence between the key plug and casing bore junction,

for the upwardly projected guard pin prevents any such relative displacement between upper and lower pin bores as would preclude pushing the key plug pins upwardly to move the upper tumbler pins into their respective casing bores. If there is uniform play throughout, the same action results.

In each instance the tumbler pins to the rear of the guard pin can be pushed upwardly to the right respective positions and when the picking tool is removed, while still maintaining through other means the torsional strain on the-key plug;

the desired extent. This may be done by using a sharp pointed instrument and by its point forcing the guard pin down. When-that is done, the key plug can be turned.

In other prior art attempts, the construction is such as to require an additional or special configuration of the key and usually, also, such structural complications of key plug and tumbler pins and arrangements that cost is prohibitive, the resultant construction impracticable, or other and more controlling deficiencies or weaknesses are brought about.

With the construction of my invention, however, such lackof security and disadvantages inherent in prior proposed constructions are not present. Becauseof the manufacturing tolerance or axial end play of several thousandths of an inch between the key plug and the casing, the diameter of the front or long pin K is illustratively 0.118 inch where the diameter of the bores PB and CB with which this pin coacts is 0.123 inch. Thus, for all normal operations on or of the lock construction, proper entry of the pin K into the upper or casing bore CB is assured in spite of such axial play between key plug and casing. Were it not for such axial play, this pin K would far more closely match the diameter of its coacting bores PB and CB. If, with the construction above described, it should become possible, through the front end of the key slot 15, to manipulate the plate P or the pin K the only possible movement that can be transmitted to the latter is an upward movement and that necessarily results in projecting'the upper end of pin K into the bore CB, with results later explained.

But it becomes exceedingly difficult to effect such a manipulation of the pin K In the first place, the plate P and the pin K are preferably not joined together mechanically so that the operator has two independently movable pieces to contend with. For example, if he is able with,

the picking tool to lift the pin K directly by operating through an upper portion of the key slot, the plate P remains bottomed in the slot 22 and continues to form a barrier to access to those tumbler pins that are tothe rear of the pin K If the plate P is displaced upwardly by a picking tool, it moves the pin K upwardly into locking position and in either case, though pin K may be held upwardlydisplaced by a torsional strain imposed upon the key plug, the same holding action is ineffective to hold the plate P displaced upwardly and the latter drops down to bottom in the slot 22. In such position it blocks off that portion of the key slot in line with the lower ends of the tumbler pins K to K and particularly that portion of the key slot above the rib l6 and below the rib I I (see Figures 2 and 3).

Assuming, however, that it were possible to achieve an upward projection of pin K and that thereafter a strain, such as a torsional strain, is imposed upon the key plug l3 so as to misaline the plug bores PB with respect to their respective casing bores, such misalinement takes place to an extent permitted by whatever differential exists between the diameter of the pin K and the diameter of its coacting bores PB and CB; in Figure 2 I have shown the pin K thus raised and have indicated a slight rotary displacement of the key plug l3 relative to the casing, such displacement being due to the presence of the dimensional differential between the diameter of pin K and the diameter of its bores PB and CB there remains only the last step which is gradually to pull the front or guard pin downwardly to necessitated by the axial end play between key plug and casing caused by the action of the parts l8 and 20, as above described. If no such axial play were present, such a dimensional differential need not be present and Figure 2 would appear without appreciable or substantial rotary displacement of the key plug relative to the casing. The same misalinement is necessarily brought about as to each remaining pair of plug bores and easing bores PB and CB, respectively, that being permitted and adequately provided for because the diameters of the pins in the remaining bores, illustratively being of a diameter of 0.115 inch, are less than the diameter of the pin K the latter being illustratively 0.118 inch.

In the illustrative dimensional embodiment,

termined by the guard pin K That restriction or throat is, however, insufiicient to prevent the free passage therethrough of any of the remaining tumbler pins so that it becomes impossible, having once raised the guard pin K as above assumed, thereafter to cause any of the pins E to E or K to K to be frictionally held or otherwise gripped or gaged'as to the location of the junction between upper and lower pins, nor is it possible to cause an upper tumbler pin E to E to be caught on any possible ledge or edge of the key plug, for the springpressure coupled with the minimum throat dimension of 0.118 inch dependably insures that the still smallerdiametered pin, namely, of 0.115 inch, freely moves downwardly through the restriction or throat in question. The physical relationship involved in these actions is shown in Figure 3.

The tumbler pin K will thus be seen to act not merely to block relative movement between key plug and casing by being projected upwardly by the inserted picking tool, but it is made, in coaction with the dimensional differentials, above illustratively set forth, to so prepare any subsequently possible restriction of the throat or passageway at the junction between upper and lower tumbler bores as to make any subsequent manipulation of key plug and other tumbler pins effective to catch or hold or position all of the remaining tumbler pins at the critical unlocking positions. The simplest mechanical embodiment for carrying out the just stated coactions is that shown in the drawing and above described in that it is possible to have uniform diameters and straight-line alinement of tumbler pin bores bodiment that is achieved by the angular relationship of the slot to the axis of the bore, as shown in Figures 1 and 4 and described above.

. This angular relationship insures that, throughout the axial length of the pin K the effective bearing surface provided by its bore PB is, as viewed in the direction of the axis of the bore, continuously cylindrical; were the axis of the slot 22 to "be coincident with the axis of the bore PB, the cylindrical surface of the bore would be interrupted on both sides and throughout its length, thus giving rise to increased play when the general positions of the parts as shown in Figure 2 are to be achieved, and such action might permit such a throat restriction as would let the smaller diametered pins be caught rather than to be freely passed therethrough, as in Figure 3.

Also, though I have shown the pin K positioned in the first tumbler recess of the key plug, it may be given a different position in the total series of tumbler pins but preferably always such a position that one or more other tumbler pins are to the rear of it.

With the above-described attempt to pick this construction failing, let it be assumed that attempt is now directed along different lines, namely, to avoid upward projection of the pin K and to attempt manipulation of the pins to the rear of pin K In such case, the plate P must be allowed to remain in its lowermost position, namely, with its lower edge 25 coinciding with the bottom 24 of the slot 22, thus closing off those portions of the key slot above the rib l6 and below the rib 11, making access to the rear pins impossible or impracticable; I have found in practice that the tortuous cross-section of the key slot above the lower edge of the plate P when it is bottomed in its slot, as just mentioned, makes it impossible to get to the rear pins and to manipulate them in the effort to get their junctions alined with the junction between key plug and casing bore. Invariably upward displacement of the plate P results if the rear pins are to be reached with any picking tool and if they are so reached and displaced, the above described coactions and results take place. Should the effort be repeated to reach a rear pin without displacing the plate P upwardly, it is to be noted that such effort is always accompanied by application of torsion or twist to the key plug l3 and due to the dimensional differentials above-described, any one of the rear pins, such as pin E .for example, permits a restriction or throat reduction throughout the construction to 0.115 inch, that being the diameter of pin E and in such case the restriction is too small to permit upward projection of pin K whose diameter is 0.118 inch, with the result that plate P becomes positively locked in the bottom of the slot 22, thus positively blocking ofi those very portions of the key slot through which access to the rear pins has to be gained.

The plate P can be inexpensively stamped out of flat stock and if desired it may be made of a metal having or treated to have sufficient hard-, ness to resist drilling. Assembly of pin K and plate P with the key plug is simple and inexpensive and the upwardly tapered or widening seat 21-46-21 permits independent movements of pin K and plate P and also conjoined movements thereof but without causing the force exerted on one to effect a binding action between the other and its guideway. Also, the plate P may be tilted as might happen if actuated by a picking tool applied to it at a point displaced from the central vertical axis, as viewed in Figure 2, in which case the plate P acts like a lever to lift the pin K fulcruming upon the bottom 24 of the slot at one or the other of the lower lateral corners of the plate.

Thus, it will be seen that there has been provided in this invention a lock construction in which the various objects hereinbeiore set forth, together with many thoroughly practical advantages, are successfully achieved. The construction will be seen to be simple, inexpensive to manufacture, and is dependably pickproof, and, moreover, it can be manufactured at low cost and made available to the consumer at a price substantially the same as or comparable to that of the ordinary or usual type of pin tumbler locks.

As many possible embodiments may be made of the above invention and as many changes might be made in the embodiment above set forth, it is to be understood that all matter hereinbefore set forth, or shown in the accompanying drawing, is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. A look construction comprising a casing and a key plug, said key plug having a key slot therein and a plurality of tumbler bores intersecting said key slot, said casing having a plurality of tumbler bores respectively alinable with said first-mentioned tumbler bores, the axes of said bores when said key plug is in position to have its bores alined respectively with the casing bores falling in a common plane, said key plug having a slot extending in a direction crosswise of the plug axis and intersecting said key slot, said slot intersecting a bore in said key plug other than the rearmost one, a plate normally bottoming in said slot and closing off at least a portion of said key slot, a tumbler pin in said intersected bore and extending upwardly beyond the upper edge of said plate, the lower edge of said plate being exposed in said bore below the lower edge of said tumbler pin, thereby to be engageable by a key or implement inserted in said key slot, said plate and said pin being of such relative lengths that, when said plate botoms in its slot, the upper end of said pin is coincident with the junction between key plug and casing, and tumbler pins in the rest of said bores including spring pressed tumbler pins in the casing bores,

2. A look construction comprising a casing and a key plug, said key plug having a key slot therein and a plurality of tumbler bores intersecting said key slot, said casing having a plurality of tumbler bores respectively alinable with said first-mentioned tumbler bores, the axes of said bores when said key plug is in position to have its bores alined respectively with the casing bores falling in a common plane, said key plug having a slot extending in a direction crosswise of the plug axis and intersecting said key slot, said slot intersecting a bore in said key plug other than the re'armost one, a plate normally bottoming in said slot and closing oil at least a portion of said key slot, a tumbler pin in said intersected bore and extending upwardly beyond the upper edge of said plate, the lower edge of said plate being exposed in said bore below the lower edge of said tumbler pin, thereby to be engageable by a key or implement inserted in said key slot, said plate and said pin being of such relative lengths that, when said plate bottoms in its slot, the upper end of said pin is coincident with the junction between key plug and easing, said tumbler pin having a diameter relative to the alinable bores in which it operates such that, when it is bridged across said junction, substantially no displacement, or only a slight displacement, be tween adjacent ends of casing and key plug bores can take place, and'tumbler pins in the rest of said bores including spring pressed tumbler pins i F 6 Cas g bores having such smaller diameters relative to their respective bores that they always freely pass across the junction between casing and key plug when said first-mentioned pin. is bridged across said junction and a displacement to the extent permitted by said pin is effective.

3. A lock construction as claimed in claim 1 in which said plate has in its upper edge a seat for the lower end of the tumbler pin, said seat comprising a cutout in the upper edge of said plate for receiving therein the lower end of said pin.

4. A look construction as claimed in claim 1 in which said plate has in its upper edge a seat for the lower end of the tumbler pin, said seat comprising a three-sided cutout in the upper elge of said plate, the bottom side being dimensioned commensurately with the lower end of the pin and the remaining two sides being inclined upwardly and away from the pin.

5. A lock construction as claimed in claim 1 in which the bore that is intersected by said slot is cylindrical and the slot intersects the bore at an angle to the axis of the bore.

6. A look construction as claimed in claim 1 in which the portion of the key slot normally blocked oif by said plate comprises a part to one side of a rib extending longitudinally of the key slot and another part extending below said rib, the remaining tumbler pins in the key plug bores terminating at or above the lower edge face of said rib.

'7. A lock construction as claimed in claim 1 in which the bottom of the slot is arcuate in substantially upwardly concave form and the bottom edge of the plate is commensurately arcuate but in substantially downwardly convex form, whereby said plate substantially self-centers itself with respect to the key slot.

8. A construction as claimed in claim 1 in which said slot is open at side portions of the key plug, said plate being of substantial dimension in a direction crosswise of the plug axis, whereby the walls of the casing bore that receives the key plug are effective to prevent substantial lateral displacement of said plate.

9. A look construction comprising a casing and a key plug, said key plug having a key slot therein and a plurality of tumbler bores intersecting said key slot, said casing having a plurality of tumbler bores respectively alinable with said first-mentioned tumbler bores, means coacting between said key plug and said casing for holding said key plug against axial movement while permitting rotary movement thereof relative to said casing, said holding means permitting a slight axial play between said key plug and casing, tumbler pins in said casing bores and tumbler pins in said key plug bores, one of said key plug tumbler pins other than the rearmost one having a length such that in normal position of the lock parts its upper end is coincident between the junction between the key plug and casing,

said one tumbler pin having a diameter slightly less than the bores within which it acts sufficient to let it enter into its casing bore in spite of said axial end play permitted by said holding means, and all of the remaining tumbler pins having diameters less than the respective bores within which they operate to an extent suflicient always to pass through the restriction effected at the junction between the key plug and the casing permitted when said one tumbler pin is projected upwardly across said junction and the key plug is displaced relative to the casing to the extent permitted by said one tumbler pin, and springs acting upon the tumblers in said casing tumbler bores to press them always through any restriction at said junction as aforesaid,

10. A construction as claimed in claim 9 in which said key slot is generally of rectangular cross-section to take a flat key but has in its walls a plurality of inwardly projecting longitudinally extending ribs received in corresponding grooves in the sides of said key, two of said ribs being vertically displaced, the key plug having a recess intersecting said key way and the bore containing said one tumbler pin, a plate in said recess terminating at or just below the upper face of the lower of said two ribs and bridging those portions of the key slot below and laterally of the upper of said two ribs, said one tumbler pin engaging the upper portion of said plate, and the bores of all such tumbler pins in the key plug as are to the rear of said one tumbler pin terminating substantially at or above the lower face of the upper of said two ribs.

11. A look construction comprising a casing and a key plug, said key plug having a key slot therein and a plurality of tumbler bores intersecting said key slot,.said casing having a plurality of tumbler bores respectively alinable with said first-mentioned tumbler bores, the axes of said bores when said key plug is in position to have its bores alined respectively with the casing bores falling in a common plane, a tumbler pin in one of said key plug bores other than the rearmost and being of a length such that in normal position of the lock parts its upper end is coincident with the junction between key plug and casing, and having a diameter relative to the alinable bores in which it operates such that, when it is bridged across said junction, substantially no displacement, or only a slight displacement between adjacent ends of casing and key plug bores can take place, and tumbler pins in the rest of said bores including spring pressed tumbler pins in the casing bores having such smaller diameters relative to their respective bores that they always freely pass across the junction between casing and key plug when said one pin is bridged across said junction and a displacement to the extent permitted by said one pin is effective.

SIGURD BRANTINGSON. 

